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<head>
    <meta charset="UTF-8">
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    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        //编写一个函数 ，输入一个数字，返还数字的长度
          var a = prompt("输入数字")
        function fn(){
            console.log(a.length)
        }
       fn(a)
    </script>
</body>
</html>